net: inline rollback_registered_many()

Similar to the change for rollback_registered() -
rollback_registered_many() was a part of unregister_netdevice_many()
minus the net_set_todo(), which is no longer needed.

Functionally this patch moves the list_empty() check back after:

	BUG_ON(dev_boot_phase);
	ASSERT_RTNL();

but I can't find any reason why that would be an issue.

Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>